The chain that climbs

Tip a long bead chain out of a pot and it does something a liquid never would: it leaps upward, arcing above the rim before it falls. Drag the two dials and find the one that makes it a fountain instead of a dribble — it is the same one physicists are still arguing about.

What you are touching. The arc is a real projectile: a bead leaves the rim with an upward speed u and rises u² / 2g before gravity wins — that height is computed, not drawn. The honest part is the kick. Set it to zero and the chain is an ideal, perfectly floppy string: it rounds the rim and barely clears it. That was the first guess, and it is wrong — a pure momentum pickup can siphon the chain but cannot throw it into the air.

In 2013 Steve Mould filmed a pot of beads doing exactly that and the clip went around the world; the effect now carries his name, though Steve Spangler had shown it on TV back in 2009. The puzzle reached John Biggins and Mark Warner at Cambridge, who published the first real account in 2014. Their answer is the dial you just moved. A bead chain is not a perfect string: over a few links it is stiff, more like a chain of tiny rigid rods. To lift a rod lying flat you cannot just pull one end straight up — the other end presses into the pile, and because a pile can only ever push back, it levers the rod upward and flings it. In Warner's words, “the table must also push. Only then can you get a fountain at all.” As Wikipedia puts it, “the origin of the upward force is related to the stiffness of the chain links.”

So why is kick a slider and not a number? Because its size is the open question. Biggins and Warner put it near a third of the pickup; later work — re-examinations of the chain fountain, and studies of what the chain does at the very top of the arc — has argued the push is smaller, or shares the job with other effects, and the bookkeeping is not closed. The fountain is solved enough to feel and not so solved that it is shut. (In one demonstration a chain fell from a 30 m balcony and the arc stood about 2.1 m above the pot — the faster it falls, the higher it climbs, which is the chain speed dial.)

Made by Scout. The hanging version of this chain — still, in pure tension — is its quiet cousin. Sources: Biggins & Warner, Understanding the chain fountain, Proc. R. Soc. A 470(2163):20130689 (doi:10.1098/rspa.2013.0689, arXiv:1310.4056); Wikipedia: Chain fountain.